1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a method of cleaning a semiconductor wafer which has a laser marking region formed at a round zone thereof.
2. Description of the Prior Art
As shown in FIG. 1A, a prior art semiconductor wafer 10 has a round zone 14a and a straight flat zone 14b. In such a wafer 10, a laser marking region 12 is provided to serve as a bar-code 12a of the wafer which has a plurality of holes or etched cavities 12b for identification. The laser marking region 12 is formed on a main surface of the wafer and in the vicinity of the flat zone 14b by a hard-marking process which uses a laser beam.
Processing high quality integrated circuits with desired yield rates from a wafer requires, inter alia, a clean wafer surface and full utilization of the wafer surface area. One of the processing problems that causes reduced yields is photoresist fragments remaining on the wafer surface after a photolithography process has been performed. Such photoresist fragments contaminate the wafer surface and cause defective integrated circuits.
As shown in FIG. 1A, a conventional photoresist 16 is applied to the center of the wafer 10 in liquid form. The wafer 10, which is vacuum mounted on a spin chuck 52, is rotated by a motor 54 to spread the photoresist material across the entire surface of the wafer 10 to form a photoresist layer 16a. See FIG. 1B. However, due to the centrifugal force created by the spinning motion, the photoresist layer 16a tends to thicken at the edges of the wafer and wrap around the edges of the wafer as shown in FIG. 1B, thereby creating an non-uniform photoresist region 16b that is inappropriate for integrated circuit patterning, which in turn, reduces the effective wafer surface utilization area.
In addition, this non-uniform photoresist region 16b tends to break-up or fragment during subsequent chemical etching processes. The photoresist fragments can contaminate the etching solution used in solution etching processes, which then contaminate the surface of a patterned wafer.
The photoresist 16 may also become lodged in the identification holes or cavities 12b of the bar code 12a of the laser marking region 12 as shown in FIG. 1C. The depth M.sub.depth of the holes or cavities may be on the order of 40 .mu.m.
Referring to FIG. 1B and FIG. 1D, there is shown a side rinse method for removing the non-uniform photoresist region 16b, and a plan view of the wafer 10 resulting therefrom, respectively. In FIG. 1B, a nozzle 30 is fixed above the wafer 10 and positioned to deliver a chemical etchant 32, such as a thinner, to the edge of the wafer 10 disposed on the spin chuck 52 while the wafer 10 is rotated by the motor 54. The etchant 32, therefore, traces a substantially uniform path 14c (dashed line) of a certain width `x` around the periphery of the wafer 10 where most of the photoresist 16b is removed.
However, in the plan view of the wafer in FIG. 1D, it is shown that a substantial portion of the laser marking region 12 is not covered by the circular cleaning path 14c due to the positioning of the laser marking region 12 in the flat zone area 14b of the wafer 10. FIG. 1E is a magnified plan view of the portion of the laser marking region 12 in the vicinity of the flat zone 14b. The chemical etchant 32 delivered during the side rinse process has failed to contact the inner portion 122 of the laser marking region 12 such that the photoresist 16 remains in the identification holes or cavities 12b in the inner portion 122. Moreover, due to the combination of the depth of the identification holes or cavities 12b and the thickness of the photoresist 16b at the edge portion of the wafer 10, some photoresist fragments 124 still remain in the outer portion 125 of the laser marking region 12 that was exposed to the chemical etchant 32.
During a subsequent etching and wet cleaning process, the photoresist material 16 lodged in the bar-code 12a of the laser marking region 12 is separated from the laser marking region 12 by contact with a solution bath thereby inducing a bubble whirl phenomenon. Bubble whirl phenomenon refers to a situation where the cleaning solution penetrates evenly through the whole surface of the silicon wafer that is dipped in a solution bath because of the presence of bubbles, which are generated from the bottom of the solution bath during the wet cleaning process and flow upwards. Unfortunately, as described above, the photoresist fragments that are separated by the solution bath become contaminating particles that are deposited on the wafer.
This contamination is illustrated in FIG. 2. While cleaning a semiconductor wafer with a wet cleaning system, particles remaining in the semiconductor wafer move from the laser marking region 12 of the wafer to the center thereof (shown as lines 13 in FIG. 2) by the bubble whirl phenomenon in the solution bath 20 of the cleaning system. More specifically, when the wafer is put in the solution bath 20 of a wafer cleaning system or a wafer stripe system in order to remove a photoresist material formed thereon, the flat zone 14b of the wafer 10 is closest to the bottom of the solution bath 20. During the cleaning process, the particles 16 separated from the holes of the laser marking region 12 move from the laser marking region to the center of the wafer 10 by the bubble whirl phenomenon.
It is known in the art that particles of photoresist fragments are a factor causing G-defects in a semiconductor wafer. G-defects mean defects occurring when particles of a photoresist material are gelatinated on the wafer surface.
During the fabrication of a semiconductor device using the above wafer 10, particularly an etching process, an EEW (edge expose wafer) system is used to prevent the clamp of the etching instrument from staining due to photoresist particles. However the particles remain lodged in the laser marking region 12, even though the particles in the flat zone 14b of the wafer 10 are sufficiently removed by using such an EEW system. This is because the clamp of the etching instrument always occupies a portion of the laser marking region 12 in the vicinity of the flat zone 14b. As a result, the remaining photoresist 16 in the holes of the laser marking region 12 serve as contaminating particles during the process that follows.
It is well known in the art that such photoresist particles are detrimental to the formation of a fine pattern, and often result in the formation of a poor pattern. Another disadvantage of locating the laser marking region 12 in the flat zone 14b of the wafer 10 is that is reduces the surface area usable for providing dies, that is, it reduces the net die area of the wafer 10.
It is desired, therefore, that the non-uniform photoresist portion be completely removed, especially in the laser marking region, prior to patterning to thereby prevent wafer defects. It is also desired to position the laser marking region on the wafer surface so as to increase the net die area.